Support strict-unsealed-patmat

[kubukoz]

In Scala 2.13, there's an -Xlint:strict-unsealed-patmat compiler flag that makes the exhaustivity checking warn in situations where you pattern match on an unsealed type.

Non-exhaustive pattern matches can be a source of countless errors, so I believe having the option to opt into such checking would be really beneficial for safety of Scala 3 code.

Can we please have such an option?